20000 US dollars for that car is what I would call well bought. You did not get the deal of the century, but you also did not get taken for a ride either. I think you paid market price for a well loved and cared for first gen Z car. You could have spent more and got less, and you could have spent less and maybe got as much or more (if you were crazy lucky)
I would not lament the purchase price at all. If the car is well sorted and all 'there' you paid a fair price for the car. Neither the sell nor yourself should have any bad feelings from the deal.
There is always the potential of finding a better 'deal', but you had very specific requirements, and with every requirement you held to (good for you by the way), the chancing of finding that 1 in a million deal dropped.
I congratulate you on a well bought car and I think you will be happy for many years to come.